A technique of high-pressure acid leach with the use of sulfuric acid has been focused as a wet smelting method for nickel oxide ore. The processing method comprises a succession of wet smelting steps without any dry processing step such as a drying step or a roasting step and will thus be advantageous for energy and cost saving and simultaneously for producing a nickel/cobalt mixture sulfide of which the content of nickel is increased to substantially 50% by weight.
More particularly, the high pressure acid leaching process for producing such a nickel/cobalt mixture sulfide includes, for example, a step (1) of subjecting nickel oxide ores to the high pressure acid leach to produce a crude nickel sulfate aqueous solution which contains zinc as a impurity element in addition to nickel and cobalt, a step (2) of introducing the crude nickel sulfate aqueous solution into a sulfurizing reaction vessel, adding the crude nickel sulfate aqueous solution with a hydrogen sulfide gas for sulfurization of zinc which is contained in the crude nickel sulfate aqueous solution, and subjecting the solution to solid/liquid separation to produce a zinc sulfide and a dezincificated end liquid, a step (3) of introducing the dezincificated end liquid into a sulfurizing reaction vessel, adding the dezincificated end liquid with a hydrogen sulfide gas for sulfurization of nickel and cobalt which are contained in the dezincificated end liquid to produce a slurry, introducing the slurry into a degassing facility for removal of the hydrogen sulfide gas, and subjecting the slurry to solid/liquid separation to produce a nickel/cobalt mixture sulfide and a process exhaust liquid, and a step (4) of purifying a remaining of the hydrogen sulfide gas contained in an exhaust gas which has been produced at both the steps (2) and (3).
The sulfurizing reaction vessels employed at the sulfurization process in the steps (2) and (3) of the high pressure acid leach are implemented by a closed type reactor vessel which has a feed inlet for introducing a reaction start liquid, a discharge outlet for discharging the slurry after the reaction, a feed inlet for supplying the hydrogen sulfide gas, and an exhaust gas outlet for discharging a gaseous component at gaseous phase as the exhaust gas.
Generally, the sulfurization process in the steps (2) and (3) is conducted by a sulfurization facility which comprises, in addition to the sulfurizing reaction vessels described above, a group of tanks including liquid storage vessels and relay vessels for storage of slurry and filtrate liquid after the sulfurizing reaction (referred to simply as liquid storage vessels hereinafter) and solid/liquid separation vessels, feed conduits for feeding the tanks with an intermediate solution such as slurry, an inactive gas, and others, discharge conduits for discharging from tanks, and piping installations for connecting between the tanks.
It has been demanded for carrying out the processing actions at higher efficiency of the steps (2) and (3) in the sulfurization facility in order to recover a wanted sulfide at a higher recovery rate as a variety of relevant techniques were proposed.
For example, disclosed in Patent Literature 1 is a method of controlling the sulfurizing reaction of metals through adjusting the concentration of hydrogen sulfide in a gaseous phase component and precisely determining the oxidation reduction potential (ORP) and the pH scale in a liquid component. Also, a method of adding a sulfide seed crystal thus to accelerating the sulfurizing reaction and simultaneously suppress the adhesion of produced sulfides to the inner surface of the sulfurizing reaction vessel is disclosed in Patent Literature 2. Moreover, a method of separating zinc at first priority through modifying the pH scales and the oxidation reduction potential (ORP) levels in a nickel sulfate aqueous solution which contains cobalt and zinc and its relation are disclosed in Patent Literature 3.
It is known in accordance with those techniques that the step (3), for example, is conducted by introducing a hydrogen sulfide gas of which the concentration of hydrogen sulfide exceeds 95% by volume into the gaseous phase compartment of a sulfurizing reaction vessel, maintaining the operational requirements, which include the concentration of nickel, the amount of supply, the temperature, and the pH scale, of a reaction start liquid having been supplied into the sulfurizing reaction vessel while controlling the inner pressure of the vessel to a predetermined degree, and adding the sulfide seed crystal if required. This results in higher than 95% of the recovery of nickel.
It may be understood for improving further the recovery of nickel at stableness that the sulfurization facility is operated with higher degrees of the temperature and the pressure at its interior. However, that situation will create a problem that the consumption of hydrogen sulfide gas and the cost for purifying an exhaust gas generated by the reactive operation or the cost for preparing the reaction vessels are increased.
Also in a hydrogen sulfide gas producing facility used in a wet smelting plant which is a practical plant for operating the high pressure acid leaching process, the production and use of hydrogen sulfide gas of which the concentration of hydrogen sulfide is lower than 100% by volume is advantageous for the production efficiency. As the result, the hydrogen sulfide gas to be added for the sulfurizing reaction contains 2 to 3% by volume of inactive components including hydrogen, which is a material for the hydrogen sulfide gas producing step, and nitrogen which is slipped into during the hydrogen sulfide gas producing step. More particularly, the hydrogen sulfide gas used for the sulfurizing reaction contains such inactive components as hydrogen and nitrogen which remain inert in the sulfurizing reaction. Consequently, during the continuous operation of the sulfurization step including the steps (2) and (3), the inactive components contained in the hydrogen sulfide gas will be accumulated in the liquid storage vessel where the slurry is stored after the sulfurizing reaction, described above, as well as in the sulfurizing reaction vessel, hence being a cause for lowering the efficiency of the sulfurizing reaction.
It is therefore essential for eliminating the above drawback to improve the efficiency of use of the hydrogen sulfide gas during the sulfurizing reaction. However, the foregoing prior arts fail to explain the improvement of the efficiency of use of the hydrogen sulfide gas.
As for this point, techniques are disclosed in Patent Literature 4, for example, where the efficiency of use of the hydrogen sulfide gas is improved by modifying the volume of the sulfurizing reaction vessel depending on the amount of nickel to be loaded and by recovering and reusing the hydrogen sulfide gas discharged from the sulfurizing reaction vessel. Those techniques allow the use of hydrogen sulfide and the use of an alkali material to be reduced while the recovery of nickel remains at higher efficiency.
However, the techniques disclosed in Patent Literature 4 fail to explain a structural drawback of installing a plurality of the sulfurizing reaction vessels needed in the sulfurization steps (2) and (3). It is hence desired for improving the efficiency of use of the hydrogen sulfide gas to propose new technologies over installation and arrangement of the sulfurization facilities.
In common practice, the sulfurization facility for conducting the prescribed steps (2) and (3) has a plurality of the liquid storage vessels where the slurry after the sulfurizing reaction is received and stored before delivering to the solid/liquid separation vessel. Also, the liquid storage vessels receive filtrates after the solid/liquid separation and repeatedly deliver those to the sulfurizing reaction vessel. The reason for employing a plurality of the liquid storage vessels is that a structural requirement exists for being implemented in some limits by an enclosure type construction and that the quantity of a target to be processed is maximized for increasing the overall production through the utilization for a variety of actions.
FIG. 4 is a structural view schematically showing a conventional liquid storage vessel 50. As shown in FIG. 4, the liquid storage vessel 50 includes an inlet conduit 51 for loading a slurry after the sulfurizing reaction and a filtrate liquid after the solid/liquid separation, a discharge conduit 52 for discharging the slurry and the filtrate liquid, an inactive gas feed conduit 53 for feeding an inactive gas (for example, nitrogen gas) which remains inert in the sulfurizing reaction, and a gas discharge conduit 54 for discharging a gas generated in the liquid storage vessel 50.
FIG. 5 is a structural view schematically showing a plurality of liquid storage vessels 50n installed in a conventional sulfurization facility for conducting the sulfurization of the above described steps (2) and (3). As shown in FIG. 5, each of the liquid storage vessels 50n includes an inactive gas feed conduit 53n and a gas discharge conduits 54n. The inactive gas feed conduit 53n and the gas discharge conduit 54n are equipped with pressure control valves 55n, 56n respectively for modifying the supply of the inactive gas received from an inactive gas feed facility or the discharge of an exhaust gas to a purifying facility thus to control the inner pressure of the liquid storage vessel 50n.
In the liquid storage vessel 50n having the foregoing arrangement, the liquid level moves up and down along the inner surface of the liquid storage vessel 50n depending on the coming in and out of the slurry or filtrate liquid during the operation and causes the pressure of a gaseous phase component to be shifted up and down. For compensation, in response to the shifting up and down of the pressure due to the moving up and down of the liquid level, the pressure control valves 55n, 56n of the liquid storage vessel 50n are controlled so as to maintain the pressure in the interior of the liquid storage vessel 50n to a constant level. More particularly, the inner pressure can be maintained to a constant level by discharging the gas accumulated in the liquid storage vessel 50n as an exhaust gas and feeding the inactive gas into the liquid storage vessel 50n.
However, the exhaust gas contains not only the inactive components but also a portion of the hydrogen sulfide gas and its discharge will result in loss of the hydrogen sulfide gas. More specifically, a portion of the hydrogen sulfide gas is removed out by vaporization from the slurry or the like stored in the liquid storage vessel 50n and then discharged as an exhaust gas, whereby the generation of a sulfide product will be declined. In particular, the liquid storage vessels 50n are installed in a group in the sulfurization facility and their action of controlling the inner pressure to determine the discharge of exhaust gases is carried out separately, whereby the total discharge of the remaining hydrogen sulfide gas as an exhaust gas will inevitably be increased thus to raise the loss of the hydrogen sulfide gas.
Moreover, the exhaust gas discharged from the liquid storage vessels 50n has to be subjected to a purifying process for removing the hydrogen sulfide gas from the exhaust gas through direct exposure to, for example, an alkali processing solution. Consequently, as the discharge of the hydrogen sulfide gas as an exhaust gas becomes high, the consumption of the alkali processing solution to be used at the step (4) will be increased.
As set forth above, the conventional sulfurization facility claims a greater loss of the hydrogen sulfide gas and it is hence desired for increasing the efficiency of use of the hydrogen sulfide gas to propose a new technology for providing an improvement of the installation and arrangement of sulfurization facilities. In particular, since the hydrogen sulfide gas is enabled to use repeatedly for the sulfurizing reaction as having stayed in the liquid storage vessels 50n where the slurry after the sulfurizing reaction and the filtrate liquid after the solid/liquid separation are stored, it is significantly needed to suppress the escape from the liquid storage vessels 50n with higher effectiveness.